REVISITING THE MILLER AND GEOLOGISTS RANGES, CENTRAL TRANSANTARCTIC MOUNTAINS: SINGLE-CRYSTAL 40AR/39AR AGES AND THEIR IMPLICATIONS

The Ross Orogen of the Transantarctic Mountains stands as the world’s longest rift shoulder. The Miller and Geologists Ranges of the Central Transantarctic Mountains expose basement rocks of the Ross Orogen and East Antarctic Craton and comprise: the Argosy Schist (with lithologies that locally include schist, quartzite, marble, gneiss, and amphibolite) and a suite of apparently consanguineous granitic plutons (the Granite Harbor intrusives). Our new data (for samples from the Polar Rock Repository of the Byrd Polar and Climate Research Center) include high resolution single-crystal ⁴⁰Ar/³⁹Ar ages of hornblende, biotite, and muscovite to provide constraints to the understanding of the tectonic and thermal history of the Argosy Schist and the cross-cutting Granite Harbor Intrusives. Using the ANIMAL facility, we find that laser single crystal ⁴⁰Ar/³⁹Ar ages of hornblende and muscovite from Argosy Schist samples range from ca. 520-490 Ma and indicate rapid post-orogenic cooling followed by a slow cooling stage of cooling through biotite closure temperatures from ca. 370-480 Ma. In addition, a granitic sample yielded biotite closure ages ranging between 454-494 Ma similar to the results obtained from samples of the Argosy Schist. These comparable ages from a range of lithologies can be interpreted to indicate they experienced a similar history of high-grade syn-metamorphic intrusion and potential polymetamorphism, which collectively contributed to a prolonged metamorphism associated with the Ross Orogeny. However, the large age spectrum of the mica phases may have also be related to the reworking caused with the regional Endurance Thrust. Substantially older hornblende and biotite ages, where present in this region, are interpreted to reflect excess ⁴⁰Ar. In addition, positively skewed probability distribution of single crystal hornblende and biotite ages that have been observed in the present study can be interpreted to indicate the effects of extraneous ⁴⁰Ar in some samples. In addition to providing updated geochronological constraints, high resolution single-crystal ⁴⁰Ar/³⁹Ar ages from the Miller and Geologists ranges enhances the understanding of the Ross Orogeny, evolution of Gondwana and potential sediment sources for glacial sedimentation.